When do you stop classifying yourself as a junior developer and make the jump forward to mid level, then onto senior? What experience level / years working in the industry

Once you want a decent salary.

@k0pernikus well, I have a job interview tomorrow, I’ve only been working commercially for a year, but I’ve been programming several years now, and I’ve been doing the job of a mid level dev on a really underpaid salary. So just tying to decide whether to ask for a bigger salary than a standard junior developer.

I don't really play this junior / medium / senior game anymore. I the end, it's an excuse to pay some poor students peanuts and as long as you can sell yourself as a seniors developer and maintain that facade, you'll get it.

I only go for "I am a developer. I want that amount of money." I don't really defend my salary in the scope "it's because of my 20-year experience in a 2-year old framework". I want it for food, rent and enjoying life. Deal with it. (And yes. I know it's a very privileged position to be in.)

Take it, make a different offer I may reject or leave it.

Then it depends on your competition. PHP pays less than TypeScript than Scala. Ask around what other people get in order to get a feeling. Don't be afraid to ask for more than you deem yourself worthy as you might still be a bargain for that company.

Compare your experience to your colleagues. If you have more, you're senior, and vice versa.